/**
  * Called by the DomainEventDispatcher when the mouse wheel has been scrolled.
  *
  * @param event The SWT event
  * @param viewer The source of the event
  */
 public void mouseWheelScrolled(Event event, EditPartViewer viewer) {
   Tool tool = getActiveTool();
   if (tool != null) tool.mouseWheelScrolled(event, viewer);
 }